1. Field of the Invention
The present invention relates to an optical glass of a lanthanum borosilicate system having a low glass transformation temperature (Tg) and excellent resistance to devitrification property, the optical glass having optical constants which are a refractive index (nd) in a range of 1.70 to 1.75 and an Abbe number (vd) in a range of 45.0 to 54.0, the optical glass being suitable for forming a glass preform material used for precision press molding, and being suitable for precision press molding.
2. Description of Related Art
Precision press molding of a glass is a technique to obtain a glass molded product having a shape of a final product or a shape extremely close to the final product and surface accuracy by performing press molding to a glass preform material, which is soften by heating, under a high temperature by using a molding die having a cavity with a predetermined shape. According to the precision press molding, it is possible to manufacture a molded product with a desired shape on the basis of high productivity without performing grinding and polishing, or hardly performing grinding and polishing after molding. Therefore, at present, glass molded products, such as spherical lenses, aspherical lenses and the like, have been manufactured by precision press molding.
Recently, miniaturizing and lightening of optical devices have been remarkably progressing. Aspherical lenses are used so as to decrease the number of lenses that construct an optical system of the optical devices. Since it is extremely difficult to manufacture an aspherical lens in large quantities and inexpensively by a method according to grinding and polishing in earlier technology, the above-described precision press molding is the most suitable molding method for manufacturing particularly an aspherical lens.
In order to obtain a glass molded product by the precision press molding, it is necessary to perform press molding to a glass preform material under a high temperature, as described above. Therefore, the molding die used in this case is also exposed to the high temperature, and a high pressure is added thereto. Thereby, in respect to the optical glass that constructs the glass preform material, it is desired to make its glass transformation temperature (Tg) as low as possible from the viewpoint of preventing the wear of the molding surface of the molding die caused by oxidation of the surface in accordance with high temperature environment and the damage of the molding surface of the molding die in accordance with the high press pressure when press molding is performed.
As a method for manufacturing a glass preform material for performing precision press molding, there is a method for manufacturing a glass preform material having a shape close to the shape of the lens, which is a final product, or having a spherical shape by obtaining a glass block cut from a glass block material, and by grinding and polishing the glass block. However, since the cutting process of the glass block material, grinding and polishing processes are required, there is a problem that those processes take time. Further, there is a method for obtaining a glass preform material by dropping or flowing down a molten glass from the tip of an efflux pipe connected to a glass melting device, forming the molten glass in a die or the like, and cooling it. In this method, cutting from a glass block material, grinding and polishing processes are not required, and it is possible to obtain the glass preform material directly from the molten glass. Therefore, as a method for manufacturing a glass preform material, the latter method is the method that has the highest mass-productivity and the manufacturing cost is the lowest. The shape of the glass preform material obtained by the latter method is a biconvex lens-like or a spherical shape. In many cases, the biconvex lens-like shape is the shape close to the shape of an aspherical lens, which is a final product, or the like. Therefore, variations in shape when precision press molding is performed can be made small, and the biconvex lens-like glass preform material has an effect of remarkably improving the mass-productivity of the lens itself. Further, when it is a spherical shape, although variations in shape when precision press molding is performed become large in many cases, there is a merit that the glass preform material can be set easily in the center of a lower die of the molding die whose molding surface is usually a concave surface.
Incidentally, in the press molding technique of glasses in earlier technology, even though devitrification was generated on a surface of a glass preform material or a pressed glass molded product, the portion of devitrification on the surface was removed by grinding or polishing performed after press molding. Therefore, it was not a problem if devitrification was not generated in the inside of the glass. However, in the precision press molding, a glass molded product to which precision pressing is performed without performing grinding and polishing or hardly performing grinding and polishing after molding is used as an optical element, such as a lens or the like. Therefore, it cannot be used as a product even if devitrification is generated only on a surface of a glass preform material or a glass molded product. That is, in the glass preform material used for precision press molding and the optical glass used for precision press molding, it is required that devitrification is not generated at a temperature suitable for forming the glass preform material, and moreover, that devitrification is not generated when precision press molding is performed to the obtained glass preform material, in addition to the glass transformation temperature (Tg) being low, as mentioned above.
Devitrification of a glass is caused when the temperature range of the glass is in the range that the nucleation temperature range and the crystal growth temperature range, which is in the high temperature side rather than the nucleation temperature range, are overlapped. The longer the time that the glass is exposed to this temperature range is, the more the crystal is grown and the devitrification progresses. In the method for manufacturing a glass preform material by dropping or flowing down a molten glass, as mentioned above, when the viscosity of the molten glass for dropping or flowing down from the tip of an efflux pipe is too low, it becomes difficult to obtain a preform material having a smooth curved surface and a spherical shape or a shape close to a biconvex lens-like shape. Further, when the viscosity of the molten glass is too high, both dropping a glass having a weight for one piece of preform material from the tip of the efflux pipe, and separating a glass gob having a weight for one piece of preform from the molten glass flow flowed down from the tip of the efflux pipe by surface tension or the like, become difficult. Therefore, it is desired to adjust the viscosity (η) of the molten glass for being dropped or flowed down in a range of logη=approximately 1.5 to approximately 2.5.
Incidentally, for the molten glass for being dropped or flowed down from the tip of the efflux pipe, it is comparatively hard to generate devitrification since the molten glass does not remain in the temperature range that the nucleation temperature range and the crystal growth temperature range are overlapped for a long time generally. However, for the glass deposited to the peripheral portion of the tip of the efflux pipe, devitrification is easily generated since it is exposed to the temperature range lower than the upper limit of the temperature range that the nucleation temperature range and the crystal growth temperature range are overlapped, that is, the temperature range that devitrification is generated, for a long time. Then, the glass in which devitrification is generated is gradually involved in the molten glass which is dropped or flowed down. Therefore, after forming of the preform material is started, devitrification becomes to be included in the glass preform material as time passes.
As a method for preventing the above-mentioned devitrification, there is a method of raising the temperature of the tip of the efflux pipe temporarily to the temperature that devitrification of the glass disappears for every predetermined time. However, as mentioned above, while the temperature of the tip of the efflux pipe is raised, the viscosity of the glass is too low, so that it is difficult to obtain a preform material having a smooth curved surface and a spherical shape or a shape close to a biconvex lens-like shape. Thereby, when the temperature of the tip of the efflux pipe is raised frequently, the productive efficiency deteriorates remarkably. Therefore, in order to manufacture a glass preform material stably and continuously, it is required to be a glass that the upper limit of the temperature range which the nucleation temperature range and the crystal growth temperature range are overlapped is as low as possible. Moreover, it is required to be a glass that devitrification is not generated even though it is kept in the temperature suitable for forming a preform material for a long time.
Further, when the precision press molding is performed by heating and softening a glass preform material, devitrification is generated since the glass preform material is exposed to the temperature range higher than the lower limit of the temperature range that the nucleation temperature range and the crystal growth temperature range are overlapped. Therefore, it is required to be a glass that the lower limit of the temperature range that the nucleation temperature range and the crystal growth temperature range are overlapped is as high as possible, and that devitrification is not generated even though it is kept in the temperature suitable for precision press molding.
The optical glass used for an aspherical lens or the like is required to have various optical constants. In particular, a glass having optical constants that are a refractive index (nd) in a range of 1.70 to 1.75 and an Abbe number (vd) in a range of about 45 to 54 is required. In earlier technology, as a glass having optical constants in the above-described ranges, a lanthanum borosilicate system glass and a lanthanum borate system glass are known as typical glasses. However, the glasses of such systems have high glass transformation temperature (Tg), and moreover, devitrification is easily generated in many cases. Therefore, from the above-mentioned reasons, a glass having a low glass transformation temperature (Tg) and excellent resistance to devitrification property is required. Further, the general standard of the superiority or inferiority in a light transmittance in an optical glass is a wavelength which shows 80% of spectral transmittance including a reflection loss in a short wavelength side. The shorter the wavelength is, the better the light transmittance is, so that it can be said that it is a glass with little coloring. However, in the lanthanum borosilicate system glass and the lanthanum borate system glass that have not less than 1.70 of refractive index (nd), the wavelength showing above-described 80% is not less than 400 nm in many cases, so that improvement in light transmittance is also required.
For example, the Japanese Patent Laid-open Publication No. Sho 54-3115 discloses an optical glass of B2O3—SiO2—La2O3—ZrO2—SnO2-bivalent metal oxide system. Although this glass has an excellent light transmittance, its glass transformation temperature (Tg) is high, so that it is unsuitable for using in precision press molding. Further, melting and refining of a lanthanum system optical glass is performed by using a melting device having a crucible or refining tank made of platinum or platinum alloy. While the glass is melted, alloying of the Pt of the crucible or the refining tank and the Sn of this glass is performed, and the alloyed portion is inferior to in heat resistance. Therefore, there may happen an accident such that a hole is opened in the crucible or the refining tank and the molten glass is flowed out from the hole. The cause of this accident is considered because the SnO2 included as an essential component in the glass is reduced to be SnO while melting or refining is performed, and moreover to Sn. Such an accident is rarely caused. However, when once occurred, economical loss is extremely large since the melting must be stopped immediately so as to dispose the melted glass by flowing it out, the melting device must be dismantled, and the alloyed platinum or platinum alloy must be recast, and then the hole-opened crucible or the refining tank must be repaired. Therefore, it must be said that this glass is unsuitable for producing safely and in large quantities by using a melting device having a crucible or a refining tank or the like, a portion which contacts with the molten glass being formed by a platinum or a platinum alloy.
Further, the Japanese Patent Laid-open Publication No. Sho 60-221338 discloses an optical glass of B2O3—La2O3—Y2O3—Li2O-bivalent metal oxide system. The object of this glass is to improve its resistance to devitrification property. However, its resistance to devitrification property is not sufficient to form a glass preform material by using the above-mentioned method of dropping or flowing down a molten glass.
Further, in earlier technology, a glass including a PbO component or a glass including a fluorine component is known as a glass having a low glass transformation temperature (Tg). However, for the glass including the PbO component, since the glass is easily fused with the die when precision press molding is performed, it is difficult to use the die repeatedly. Therefore, it is unsuitable for using in precision press molding. Further, for the glass including the fluorine component, when forming of a preform material is performed, the fluorine component is selectively volatilized from the surface layer of the molten glass and clouds are generated in the preform material, or the fluorine component is volatilized and deposited to the die when precision press molding is performed to the preform material, and clouds are generated on the surface of the die or the surface of the glass molded product to which the precision press molding is performed. For these reasons or the like, it is not suitable as a glass for manufacturing a glass preform material for using in precision press molding or as a glass for using in precision press molding.